The rise of C4 grasses in South America: Linking grassland transition to the south american summer monsoon

Abstract

The expansion of C4 grasses is one of the most dramatic ecological changes in the past 65 million years. Beginning in the Late Miocene (~7 Ma), these tropical and subtropical grasses began to spread and now cover roughly 25% of the Earth's surface. C4 grasses include economically important crops, but the environmental conditions that drove this global expansion are poorly understood. In this study, we aim to determine the drivers of C4 grass expansion in South America. We hypothesize that this expansion was driven by the strengthening of the South American Summer Monsoon (SASM), which occurred as a result of regional climatic change due to local tectonic and global climatic changes. Using carbon isotopes of bulk organic matter preserved in paleosols, we reconstruct the abundance of C4 grasses across 7 sites in Argentina to assess the timing of this ecological transition in South America. We pair this C4 grass reconstruction with phytolith and biomarker analysis to gain a more detailed perspective of vegetation and fire history, and we use soil geochemistry proxies to assess changes in hydroclimate across the region from the Late Miocene-Pliocene. Our data shows the largest Miocene abundances of C4 grasses in the Northwest Argentina, in the Subandean ranges of Salta and Tucumán, as well as in exposed sediments in the western Chaco plain in Santiago del Estero and La Pampa. C4 abundances are lower in central Argentina in Mendoza, similar to those observed in the modern day. We find that in the absence of temperature and atmospheric CO2 changes, hydrologic change is likely the driver of C4 grass abundances. These results are supported by Community Earth System Model simulations that suggest an increase in summer precipitation along the eastern foothills of the Andes in Argentina between 8 and 3 Ma. Our data does not show a correlation between fire frequency and C4 grass abundance in Argentina, unlike in the Siwaliks of South Asia. We also note that we only find moderate to high abundances of C4 grasses in areas where proxy-based precipitation estimates exceed ~500mm/yr. We do not observe C4 grasses at sites with modern elevations higher than ~3,000 feet or behind rain shadows, showing that tectonically-driven climate changes are important controls on the distribution and spread of C4 grasses in South America. This work will allow us to better predict changes to modern grass and crop-lands in the future.